Proposal: Add conspicuously missing Functor instances for tuples

[Christopher Allen]

I've addressed this here:

http://bitemyapp.com/posts/2015-10-19-either-is-not-arbitrary.html

The thousand-papercuts opposition to typeclass instances on the premise
that a Functor for (a, b, c) maps over the final type not making sense is a
rejection of how higher kinded types and typeclasses work together. This is
natural and predictable if one bothers to explain it.


On Mon, Jan 18, 2016 at 2:42 PM, Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Mon, 18 Jan 2016, David Feuer wrote:
>
> Actually, I currently get a much worse error message:
>>
>> Prelude> fmap (+1) (1,2,3)
>>
>> <interactive>:2:1:
>>    Non type-variable argument in the constraint: Functor ((,,) t t)
>>    (Use FlexibleContexts to permit this)
>>    When checking that ‘it’ has the inferred type
>>      it :: forall b t t1.
>>            (Functor ((,,) t t1), Num b, Num t, Num t1) =>
>>            (t, t1, b)
>>
>> That there is a *lousy* error message.
>>
>
> indeed
>
>
> I understand that there are newcomers who struggle to understand the
>> difference between tuples and lists; I don't think that's a good
>> enough reason to omit a good and valid instance.
>>
>
> I would not qualify myself as a newcomer. For me these instances are a
> problem, not a help.
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>


-- 
Chris Allen
Currently working on http://haskellbook.com
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